A. Field of the Invention
This invention relates to a method of treating a subterranean formation containing sandstone and clay components and penetrated by a well bore and more specifically, it pertains to a method of fracturing such a formation with an acid to effect an increase in permeability of the formation.
B. Description of the Prior Art
Acid treating or acidizing of porous subterranean formations penetrated by a well bore has been widely employed for increasing the production of fluids, e.g., crude oil, natural gas, etc., from said formations. The usual technique of acidizing a formation comprises introducing a non-oxidizing acid into the well under sufficient pressure to force the acid out into the formation where it reacts with the acidsoluble components of the formation. Hydrofluoric acid or a mixture of hydrofluoric acid and hydrochloric acid, commonly known as "mud acid" is typically employed to dissolve silica and clay.
During the acid treating operation, passageways for fluid flow are created in the formation, or existing passageways therein are enlarged, thus stimulating the production of fluids from the formation. This action of the acid on the formation is often called etching. Acid treating or acidizing operations wherein the acid is injected into the formation at a pressure or rate insufficient to create cracks or fractures in the formation is usually referred to as matrix-acidizing.
Hydraulic fracturing is also commonly employed to increase the production of fluids from subterranean formations. Hydraulic fracturing comprises the injection of a suitable fracturing fluid down a well penetrating a formation and into said formation under sufficient pressure to overcome the pressure exerted by the overburden. This results in creating a crack or fracture in the formation to provide a passageway which facilitates the flow of fluids through the formation and into the well. Combination fracture-acidizing processes are well known in the art.
In a typical fracturing treatment, a particulate propping agent, e.g., sand, glass beads, sintered bauxite, nut shells, zirconia, alumina, and the like, is placed in the fracture to keep the fracture at least partially propped open.
It is well known that hydrofluoric acid rapidly dissolves silica, clay, and mixtures of silica and clay. For example, see U.S. Pat. No. 4,056,146 which is assigned to the assignee of the present invention and is herein incorporated by reference. This knowledge has been utilized in the acidizing of subterranean formations containing silica and clay to increase the permeability of such formations whereby the ability of fluids, such as oil, gas, and water to flow through the thus treated formations is enhanced.
One of the problems commonly encountered in a fracturing process is that the proppant pack or fracture face is frequently less permeable than it might be due to the presence of fines dispersed in the proppant pack or fracture face. The term "fines" as used herein shall include siliceous fines, clay platelets, and other similar fines. For purposes of this invention, clay is a hydrated aluminum silicate. The presence of these fines results in less than optimum fracture conductivity. The fines may be initially present in the proppant or fracture face, or may result from the migration of fines from the formation into the proppant pack or fracture face. Regardless of the source of the fines, and regardless of whether the fines are located in the proppant pack or in the fracture face, it is desirable during a fracturing operation to increase the fracture conductivity by removing the fines from the proppant pack and/or fracture face of the formation.